But before examining the scientific accuracy of Neil's latest comments, given that his show is a political one, let's first talk about climate policy. On that subject, Neil explained why he focused on the so-called 'pause' in global surface warming during his show:

"it is legitimate to ask if the government takes the pause seriously and if it has any implications for policy ie, if there is a pause in warming, is there a case for the government to pause or slowdown its expensive efforts to decarbonise the economy until the picture becomes clearer?"

That certainly is a legitimate question. In fact, it's a question that a number of climate scientists have answered.

The question is based on the fact that a few recent studies have concluded that the sensitivity of the Earth's climate to the increased greenhouse effect may be slightly less than previous best estimates, based on the recent slowed surface warming (for a basic primer on climate sensitivity, see my previous entry here). There are reasons to be skeptical of this conclusion, but it's certainly a possibility. If true, would that mean governments should pause or slow down their efforts to decarbonize the economy, as Neil asks?

In short, if these studies are right, it might take us an extra decade or so to reach global warming levels considered unacceptably dangerous. If true, that would certainly be welcome news. The problem is that from a policy perspective today, these sorts of details don't matter.

In order to avoid what's internationally considered dangerous levels of global warming, we need to achieve tremendous levels of greenhouse gas emissions reductions. Developed nations like the UK and USA will need to reduce their emissions by about 80 percent by the year 2050 to give us a good shot to avoid committing the planet to 2°C average surface warming, and even that is considered too risky by many climate scientists.

We should certainly fashion a climate policy that will maximize the economic benefit associated with reducing greenhouse gas emissions, and what that policy should look like is a valid debate in which all who would like to participate are more than welcome. However, there is no question as to whether we can 'slow down' our efforts – we simply cannot afford to, even in the best-case scenario.

This graph shows that even at the lowest range of climate sensitivity, future global warming will take us well beyond any temperature experienced during civilized human history. The blue line represents reconstructed temperature. The red line represents measured and projected global surface temperature. The red dots show the projected warming in the year 2100 for three different climate sensitivities (high sensitivity 4.5°C, most likely sensitivity 3°C, low sensitivity 1.5°C).

Now, on to the science.

The "Pause"

Neil explained that he focused on the surface warming 'pause' because he was trying to challenge Davey to defend the government's climate policy in the face of this seemingly contradictory global warming evidence. The problem is that when Davey correctly pointed out that surface temperatures are only one small piece of overall global warming (about 2 percent), and melting ice and warming oceans must also be considered (over 90 percent of the overall heating of the planet), Neil remained focused exclusively on surface temperatures.

In science, and in informed policy making, you can't just ignore 98 percent of the evidence and focus on the 2 percent that seems to support the argument you're trying to make. Perhaps Neil was just doing his job "challenging" Davey, but in terms of a science and policy debate, Davey won the day by considering the full body of evidence.

The Faulty Temperature Graphic

In defending his use of an overly smoothed surface temperature graphic, Neil explained,

"The graph we presented illustrating the temperature plateau was not constructed by the Sunday Politics but taken from a website, produced by Phil Jones, a leading figure at the Climate Research Unit, University of East Anglia"

That is true - you can view the graphic in question here. Phil Jones should have explained on that page exactly what the black line represents, and that the smoothed data are not accurate after 2007 (or 2002, depending on whether it's a 10- or 20-year smoothing), as I explained in my last entry. Whoever is at fault, the temperature graph presented on BBC Sunday Politics was not accurate, and exaggerated the surface warming slowdown.

Frequent Plateaus

Neil responded to my point that there have been many similar 'plateaus' in the surface temperature record as follows.

"Mr Nuccitelli points out that temperatures have plateaued in the past, which is true. But since that was before, according to the IPCC, global warming became the dominant factor in temperature rises, it is not clear past plateaux [sic] are relevant to this debate"

This is incorrect. As Neil notes, the IPCC says that greenhouse gases became the dominant factor in global temperature changes in the 1970s. My graphic begins in 1970 and shows four similar 'plateaus' since then, prior to the current one.

Ocean Warming

On the rapid warming of the oceans, Neil claimed,

"There is a huge debate in climate science over the relationship between global warming and ocean temperatures. As pointed out above some scientists (and Mr Nuccitelli) believe that global warming is causing the depths of the oceans to heat up and that one day this heat will be released.

This is widely contested and even, by some, dismissed. The data is short-lived and contentious"

The transfer of heat to the deep oceans has indeed accelerated over the past decade, but research indicates that this is a temporary change. I summarized the recent research on this subject here, and is also the conclusion in ongoing research by the Met Office, as their scientists discussed in a press briefing yesterday.

The ocean measurements quite clearly show that global warming continues at a rapid rate, equivalent to 4 Hiroshima atomic bomb detonations per second.

Global heat accumulation from Nuccitelli et al. (2012)

While ocean heating research is ongoing, the evidence points strongly in the direction that the oceans are absorbing more heat, contributing to the slowed surface warming, but that this is only a temporary effect.

Human-Caused Arctic Sea Ice Death Spiral

Neil also downplayed the human role in the rapid Arctic sea ice decline, which has seen a loss of three quarters of its ice mass over the past three decades.

"Others point out that satellite observations began in 1979 and caught a decline in Arctic ice already in progress. So the origin of the decline could be many decades ago, and might not have been started by man (though global warming could now be exacerbating a previous "natural" melting trend)."

One of the most widely used long-term estimates of Arctic sea ice extent comes from the University of Illinois. A description of the vast array of measurements used to create this estimate is available here, and the data are plotted below. As you can see, the vast majority of the Arctic sea ice decline has happened since 1970.

Average July through September Arctic sea ice extent 1870–2008 from the University of Illinois and observational data from the National Snow and Ice Data Center for 2009–2012

I have also summarized the research about the causes of that decline, which shows that humans played a dominant role. For example, a 2012 study published in Environmental Research Letters estimated that only 5 to 30 percent of the Arctic sea ice decline from 1979 to 2010 could be attributed to natural cycles, and even more is human-caused if we extend the analysis as far back as the 1950s, because the influences of natural cycles tend to average out to zero over time.

The bottom line is that Arctic sea ice is declining exceptionally rapidly, and humans are the dominant cause.

Danish Meteorological Institute (DMI) Report

Neil cited a DMI report to argue otherwise.

"A new paper by the Danish Meteorological Institute (DMI) suggest that Greenland ice sheet melting is related to solar activity and "a considerable fraction of the current withdrawal could be a natural occurrence"."

First of all, this isn't a paper published in a peer-reviewed journal, it's a report issued by the DMI Ministry of Transport. Second, it's not new - it was published in 2005. However, for unknown reasons, some climate contrarian blogs recently decided to report on this study, which is undoubtedly why Neil thought this report was new. This reminds me of a comment I made in my previous article,

"If Neil relies on contrarian blogs for his climate information, that may explain why he is woefully misinformed on the subject."

The DMI report itself is simply based on correlations, and the comment about the current ice withdrawal highly speculative. I don't find it convincing for reasons discussed here and here, for those who are interested. In any case, if 5 to 30 percent of the current decline is considered "a considerable fraction," then it is consistent with the findings of Environmental Research Letters paper discussed above.

The 97 Percent Human-Caused Global Warming Consensus

Finally, Neil again discussed the paper I co-authored finding a 97 percent consensus in the peer-reviewed literature that humans are causing global warming. Neil backtracked from his previous (false) claim that our paper had been "substantially discredited" to now say it "is not uncontested." This is a rather meaningless statement - I would challenge Neil to find any scientific paper that is uncontested.

"Well, it turns out that the 97% consensus that they found, I am indeed part of ... What do all those people agree to? Well, they agree to something fairly innocuous and it's something most of us agree to. That humans must have some influence on climate."

This statement is wrong because it misses the nuance in our study. The "skeptic" papers included those that rejected human-caused global warming and those that minimized the human influence. Since we made all of our data available to the public, you can see our ratings of Spencer's abstracts here. Five of his papers were captured in our literature search; we categorized four as 'no opinion' on the cause of global warming, and one as implicitly minimizing the human influence.

Thus, contrary to his testimony, Spencer was not included in the 97 percent consensus. In fact his research was included in the fewer than 3 percent of papers that either rejected or minimized the human contribution to global warming.

Our survey also included categories for papers that quantified the human contribution to global warming. In the author self-ratings phase of our study, 237 papers fell into these categories. 96 percent of these said that humans are the primary cause of the observed global warming since 1950. The consensus on human-caused global warming is robust.

A Policy Discussion Worth Having

To sum up, there's nothing wrong with having a discussion about what the best climate policy would entail. In fact, this sort of discussion would be constructive and welcome. But it must consider all the available evidence, and it must get the science right, or the policy discussion will be misinformed.

The scientific evidence indicates that the climate is warming rapidly due to human greenhouse gas emissions. If we want to avoid dangerous and potentially catastrophic climate change, we need to take major steps to reduce our greenhouse gas emissions. The question is not whether we should slow down our efforts; it's how we can speed them up in the most economically beneficial way. Now that is a policy discussion worth having.